There's a program called "winsetup.exe" that should be in the Steam folder for the game. Give it a try and let me know if it works. (For example, 2x or 3x or 4x nearest neighbor will get the size up.) Sometimes the other resolution modes may cause their own issues, though.

Incidentally, the pixels will always be chunky -- we're not talking about higher resolution in that sense, just in terms of a larger window if you're in windowed mode, or possibly different performance in full screen.